The present invention is directed to a method of on-line control of the opacity of a moving sheet material and more particularly to a method where the basis weight of the material varies and where an opacity additive also varies basis weight.
Previous on-line opacity control systems have found only limited use because the control of expensive additives on the basis of opacity measurement alone had not proved to be an efficient or profitable technique. Such control systems have been based on the premise of excellent basis weight control. Moreover, such methods have been unworkable for target shifts or grade changes since whenever basis weight would be changed it would cause a waste of expensive additives or the production of substandard sheet materials. In addition, the use of independent closed loop control systems for both opacity and basis weight tended to produce oscillations in such control loops after every change in opacity target or basis weight target. In order to reduce such oscillations one would typically tune the controllers for slow response which leads to lower quality of control for fast transients.